1. Field of the Invention
The invention relates to a phenolic compound-modified spent sulfite liquor, such as a phenol-modified spent sulfite liquor, and to a process for preparing the same. The invention also relates to a thermosetting resin formulation which contains the phenolic compound-modified spent sulfite liquor.
2. Description of the Prior Art
Spent sulfite liquors are the by-product of sulfite pulping processes by which wood lignin is modified with sulfonic groups to render the lignin water soluble. They are commercially available in liquid and powder forms at relatively low cost. The spent sulfite liquor contains lignosulfonate, wood sugars, resins, organic acids and their residual pulping chemicals. Lignosulfonate and wood sugars represent about 50%-65% and 10%-25% on dry basis, respectively, and are the major components of spent sulfite liquor. Depending on the bases used for the original cooking chemical, the spent sulfite liquor contains ammonium, sodium, magnesium or calcium lignosulfonate.
Although spent sulfite liquors are low cost, they are seldom used for thermosetting resin formulations. The spent sulfite liquors are not used because of their limitations, which are: (1) the lignosulfonate molecules have insufficient reaction sites to react with formaldehyde and/or to cross-link with phenol-formaldehyde resin; (2) the cured resin has a high hydrophilic property due to the high content of sulfonic acid groups, and (3) the cured resin is vulnerable to biological degradation since the commercial spent sulfite liquors contain a high content of undesirable sugars.
Spent sulfite liquor may be used as an extender for phenol-formaldehyde resins. A mixture of spent sulfite liquor with a phenol-formaldehyde resin or a resin prepared by mixing spent sulfite liquor with phenol and then reacting it with formaldehyde has been suggested for adhesives to be used for wood product manufacture. However, the adhesion property of these resins are inferior to the conventional phenol-formaldehyde resins as a result of the undesired properties of the spent sulfite liquor previously described.
In order to increase the formaldehyde reactivity of spent sulfite liquor, the prior art has suggested modifying the lignosulfonate by condensing it with phenol. However, the condensation reaction is usually hampered since the phenol reacting sites of the lignin are preoccupied with sulfonic acid groups. Thus, a strong mineral catalyst, such as hydrochloric acid, and a high temperature, generally greater than 140.degree. C., are required to induce the reaction. The disadvantage of the process is that it requires an acid-resistant and/or pressurized reactor. Moreover, the spent sulfite liquor may be condensed to such a degree that it is unable to dissolve in alkaline solution, and thus is not suitable for resin formulation.
For example, U.S. Pat. No. 2,385,586 describes the reaction of a solution or dried powder of spent sulfite liquor with a phenolic compound in a mineral acid at a pH of less than 4 and at a temperature of 50.degree. C. to 150.degree. C. Similarly, U.S. Pat. No. 2,772,139 discloses the reaction of a calcium-based spent sulfite liquor having 10%-50% solids content with sulfuric acid to remove the calcium, and then reacting the mixture with an excess of phenolic compound at 100.degree. C. to 150.degree. C. In each instance, a sealed, acid-resistant autoclave is required.
U.S. Pat. No. 2,772,140 describes the removal of calcium ions from a calcium-based spent sulfite liquor having 10%-50% solids content by ion exchange and then reacting it with an excess of phenolic compound at 100.degree. C. to 150.degree. C. by reflux or in a sealed, acid-resistant autoclave. The resulting product, containing a phenol derivative of a spent sulfite liquor and unreacted phenol, can be used to prepare a thermosetting resin by reacting it with an aldehyde as described in U.S. Pat. No. 2,794,790. The resulting resin may be used as a binder in the manufacture of wood products. The formation of the phenol derivative is costly because of the ion exchange step and the need for the acid-resistant autoclave.
This latter disadvantage is also present in the process of preparing a phenol-reacted spent sulfite liquor as described in Chemical Abstracts 80:P147112Y. In this process, concentrated spent sulfite liquor is reacted with phenol at 160.degree. C. to 200.degree. C. and at a pH of 5.1-7.2 in an autoclave. A water-resistant adhesive can be prepared by reacting the product with an aldehyde in an alkaline solution.
Finally, U.S. Pat. No. 4,127,544 discloses a phenol-reacted, ammonium-based lignosulfonate prepared by dissolving or suspending a solid, ammonium-based spent sulfite liquor in phenol and heating the mixture to 150.degree. C. to 300.degree. C. under autogenous pressure until the reacted mass is water-insoluble but soluble in aqueous sodium hydroxide. The ratio of ammonium lignosulfonate to phenol may be 2:1-1:10. The resulting product may be reacted with an aldehyde to make an exterior grade resin adhesive.
The prior art also further shows the reaction of phenol with lignosulfonate at an alkaline pH. U.S. Pat. Nos. 3,597,375 and 3,658,638 disclose heating an alkali metal lignosulfonate with phenol at 90.degree. C. to 180.degree. C. and at pH 9-12. The product can then be reacted with an aldehyde under alkaline conditions to form an adhesive useful for plywood manufacture. This process, however, suffers from the following disadvantages. The sugars which are present in the spent sulfite liquor and which are undesirable for resin formulations are not decreased in concentration by the reaction at a highly alkaline pH. In addition, the reaction requires the spent sulfite liquor be converted to sodium-based spent sulfite liquor.
Sakanen, K. V., 27th Int. Cong. of Pure & Applied Chem., Varmavuori, A., Ed., Pergamon Press, Oxford, pp. 299-306 (1980) is a review of lignin and phenolic polymers. The reaction of lignin sulfonates with phenol at acid, neutral and basic pH and at temperatures of less than 200.degree. C. is described. It was determined that phenol does not replace the sulfonate groups at near-neutral pH.
An additional reaction of spent sulfite liquors or lignosulfonates which has been described in the prior art is the oxidation of these materials. U.S. Pat. No. 2,470,764 describes the oxidation of spent sulfite liquor using an oxidizing agent, such as hydrogen peroxide. The use of hydrogen peroxide or persulfates of oxidize spent sulfite liquor under acidic conditions is disclosed in U.S. Pat. No. 3,388,061. This oxidation can be carried out to form a free-flowing powder as disclosed in U.S. Pat. Nos. 3,476,740 and 3,544,460.
Chemical Abstracts 98:73516f (1982) describes the oxidation of spent sulfite liquor with air or oxygen at 140.degree. C. to 180.degree. C. until a pH of 3-6 has been obtained. The product is then reacted with phenol and formaldehyde to form an adhesive for wood laminates. Nimz, H. H. et al, Applied Polymer Symposium, No. 28, John Wiley & Sons, Inc., pp. 1225-1230 (1976) disclose the oxidation of spent sulfite liquor under neutral conditions with the use of hydrogen peroxide and potassium ferricyanide. The oxidation generates phenoxy radicals which couple to cross-link the spent sulfite liquor.
Desulfonation and dimethylation can occur if the lignosulfonates are treated with caustic alkali at elevated temperatures. U.S. Pat. No. 3,551,405 discloses that this reaction with spent sulfite liquid results in desulfonation, to produce a desulfonated lignin resin. U.S. Pat. Nos. 2,491,832 and 4,219,471 disclose that this reaction in the presence of air results in an oxidized, partially desulfonated product. U.S. Pat. No. 2,505,304 discloses that the alkaline hydrolysis of lignosulfonate under pressure and at elevated temperatures causes demethylation.
U.S. Pat. No. 2,849,314 describes a process of treating spent sulfite liquor to produce reactive nitrogen-containing compositions capable of condensation with reactive aldehydes to form thermosetting resins. The process comprises removing inorganically combined sulfur from the liquor with addition of lime, heating the liquor from about 275.degree. F. to 350.degree. F. in the presence of ammonia and an oxygen-containing gas for a sufficient time to impart a substantial nitrogen content to an acid precipitable component. The oxygen-containing gas is utilized to oxidize the sugars present in the spent sulfite liquor.
Although the prior art describes many processes for treating spent sulfite liquor in an attempt to use the treated liquor for binders or adhesives, the products still suffer from one or more of the disadvantages described above. For example, if the spent sulfite liquor is desulfonated and oxidized to reduce the sugars content, the treated liquor still contains insufficient reaction sites to react with an aldehyde and/or to cross-link with a phenol-aldehyde resin. The present invention obviates the disadvantages of the prior art.
The invention provides a new and economical process for manufacturing phenol-reacted spent sulfite liquor. The reaction can be carried out at moderate acidity and temperature. Therefore, the process does not require acid-resistant and pressure equipment. The process reduces a substantial amount of the sulfonic acid groups from the molecule of lignosulfonate so that the cured resin is durable and has a low hydrophilic property. The process further reduces a substantial amount of the sugars present in the spent sulfite liquor, so that the cured resin is invulnerable to biological degradation. Finally, the phenol-reacted spent sulfite liquor is reactive with aldehyde and able to cross-link with phenol-aldehyde resins. It can be used for a thermosetting resin formulation by further reacting with aldehyde, and it functions as a reactant. Consequently, a substantial amount of phenol can be replaced with the phenol-reacted spent sulfite liquor in a resin formulation.